In this paper an efficient method to study sound generation processes in low Mach number flows is presented. We apply the methodology on a two-dimensional flow over a cavity with smoothed corners. Instead of the full flow field obtained from, for example a Direct Numerical Simulation (DNS), we use a reduced model based on global modes to obtain the aeroacoustic sources. Global modes are eigenmodes to the Navier-Stokes equations, linearized about a steady base flow. In a reduced model the perturbations from a steady state are approximated by a linear combination of the eigenmodes. The time dependence is determined by the corresponding eigenvalues. Curie's equation is used to calculate the acoustic field, and by studying the source terms in Curie's equation, mechanisms for sources of sound are identified. Results of acoustic pressure in the far-field and source strengths for different superpositions of eigenmodes are presented.

Aeroacoustic calculations are performed for a two-dimensional configuration of an orifice plate mounted in a straight duct in a low Mach number flow. The flow field is calculated by solving the compressible Navier-Stokes equations by means of a direct numerical simulation, using a high order finite difference scheme. The scheme is based on summation by parts operators and a penalty techniques is used to impose boundary conditions. Methods to decompose an acoustic field into upstream and downstream propagating waves are investigated for use in flows with vorticial structures present. The investigated methods are one formulation of a Two microphone method, and one technique which utilize the relation between acoustic pressure and velocity in an acoustic plane wave.

In this paper we present calculations of sound wave propagation in a straight duct with an orifice plate and a mean flow present. The wave propagation is modelled with a frequency domain linearized Navier-Stokes equations methodology. A two-dimensional approximation is used to an axisymmetric cylindrical geometry, and an appropriate frequency scaling is utilized to account for this. The relation between pressure and density is assumed isentropic and correction for duct damping based on viscous dissipation in the acoustic boundary layers is applied. Calculations are carried out for frequencies in the plane wave range up to the cut-on frequency of the first higher order propagating acoustical mode, and performed with a commercial Finite Element Method code on a quadrilateral mesh with third order shape functions. Results of transmission through, and reflections at the orifice are presented on a two-port scattering matrix form and are compared to measurements with good agreement.